1. Field of the Invention
The present invention relates to a method of manufacturing a porous metal sheet which is preferably used as an electrode substrate of a battery; the porous metal sheet manufactured by the manufacturing method; and an electrode for a battery using the porous metal sheet. More particularly, the present invention relates to a porous sheet formed of metal powders so that an active substance is filled into pores thereof. The porous metal sheet is preferably used as the electrode substrate of a nickel hydrogen battery, a nickel cadmium battery, a lithium primary battery, a lithium secondary battery, an alkali dry cell, a fuel cell and so on; and an electrode plate of various batteries, for example a battery for vehicles.
2. Description of the Related Art
As porous metal sheets of this type which are used as the electrode substrate of batteries, the present applicant proposed various kinds of metal porous materials made of a foamed material, a nonwoven sheet, a mesh material, a laminated sheet comprising two or more thereof by treating these materials so that they are electrically conductive and then electroplating them.
In manufacturing the porous metal sheet by the method, it is necessary to perform an electric conductive treatment by a method such as vaporizing method, chemical plating method or carbon application method as a pre-treatment of electric plating. It is troublesome and costly to carry out these methods. Further, when the foamed material, the nonwoven sheet, the mesh material are electroplated and burnt to remove resinous material and metal powders are sintered, burnt-off portions are cavitied. Thus, an active substance cannot be filled thereinto.
In view of the problems, the present applicant proposed many methods of manufacturing the porous metal sheet from metal powders.
In any of the above-described methods, using adhesive agent, fine metal powders are applied to entire surfaces including the inner surfaces of pores of the foamed material, the nonwoven sheet, the mesh material or the laminated sheets thereof so as to form a conductive metal layer thereon. Then, the resinous material is removed and the metal powders are sintered to form porous metal sheets.
In the above-described conventional methods of forming the porous metal sheet, using fine metal powders, fine metal powders are applied to the surface of the porous base plate such as the foamed material, the nonwoven sheet, the mesh material, or the laminated sheets thereof. Thus, the size and shape of the pore of the porous metal sheet is restricted by the size and shape of the pore of the porous base plate. Thus it is difficult to form a pore smaller or greater than the pore of the porous base plate and a pore whose shape is different from that of the pore of the porous base plate.
As one of the conditions required as the substrate of the electrode plate, it is necessary that the porous metal sheet is thin to accommodate a large amount of an active substance so as to improve the performance of a battery. But in the above-described conventional methods of forming the porous metal sheet, using fine metal powders, the thickness of a porous metal sheet is restricted by the thickness of the base plate. Hence it is difficult to manufacture a porous metal sheet having a thickness 1 mm or less.
Further, adhesive agent is used in any of the above-described conventional methods. That is, fine metal powders are applied to the base plate by mixing them with the adhesive agent or after the adhesive agent is applied to the base plate. Therefore, the adhesive agent is present between adjacent fine metal powders. Thus when the adhesive agent is burnt off, together with the base plate during the removal of the resinous material and the sintering of the fine metal powders, large gaps are formed between the adjacent fine metal powders. As such, it is difficult to control the shape and size of the pore. In addition, the number of processes is increased because the adhesive agent is used.
Further, a solid metal foil is hitherto used as the substrate of the positive and negative electrodes of a lithium secondary battery. In this case, lithium ions are incapable of moving from the front surface of the electrode substrate to the rear surface thereof and from the rear surface thereof to the front surface. Therefore, in order to obtain a possible most uniform and thinnest active substance layer, the active substance is required to be applied to one surface of each of the positive and negative electrodes. In addition, because the surface of the substrate is smooth, the active substance is liable to be separated from the base plate.
In porous metal sheet having punching shape, lath-shape, mesh-shape, foamed sheet-shape, nonwoven sheet-shape and so on, lithium ions are capable of moving from the front surface of the electrode substrate to the rear surface thereof and vice versa and further, the thickness of the active substance can be controlled at both the front and rear surfaces of the electrode substrate by the porous metal sheets. Thus, research and development are being made to use such the porous metal sheets as the electrode substrate of the lithium secondary battery. But the size of the solid portion of the conventional porous metal sheet and the size of the pore thereof are not uniform. Therefore, the lithium ions are incapable of moving uniformly and sufficiently. Although it is preferable that the porous metal sheet has a large number of small pores thereon to allow the lithium ions to move smoothly, the porous metal sheet which satisfy such a demand has not been proposed.
The electrode substrate of the lithium secondary battery is required to have a thickness of 10 .mu.m-30 .mu.m. But as described above, it is difficult for the conventional art to manufacture a porous metal sheet having a thickness 1 mm or less. That is, it is impossible to manufacture a porous metal sheet whose thickness is as small as a thin foil.
In recent years, portable equipment such as a video camera, a liquid crystal compact television, a CD player, and the like requiring high current have come into wide use. Thus, there is a growing demand for the development of batteries having a great discharge capacity and superior in discharge characteristic in a high load-applied state. But in the conventional alkali dry cell having a separator, pellets having positive electrode is filled outside the separator and gel powdered zinc is filled inside the separator. As such, it is very difficult for the alkali dry cell to have a great discharge capacity and have an improved discharge characteristic in a high load-applied state because the battery can has a limited space.
In order to solve the problem, researches are made on an alkali dry cell whose negative electrode plate consists of a punching or expanding solid zinc foil or zinc foil and positive electrode plate consists of metal oxide. The negative electrode plate and the positive electrode plate are spirally coiled with a separator interposed to increase the area of the negative electrode plate and the positive electrode plate and the discharge capacity of the battery and improve the discharge performance thereof in a high load-applied state. But the punching or expanding zinc foil has a problem that the open area ratio is about 50% or less because pores are two-dimensional; pore-forming processing is carried out and thus a pore-formed portion is cut off, and hence material left parts of material are much; processing and material costs become high as the thickness of the zinc foil is increasingly thin; and strain and burr are liable to appear in the pore-forming process. Further, the solid zinc foil and the conventional porous metal sheet have problems similar to those of the lithium secondary battery.
Further, conventionally, the electrode of alkali secondary batteries such as the nickel hydrogen battery and the nickel cadmium battery is formed as follows; paste-like slurry of an active substance formed by mixing an active substance such as hydrogen-storing alloy powder or powder of nickel hydroxide and electrically conductive agent comprising carbon, binder and so on is applied to a collector such as punching metal, metal mesh, expanded metal. But the binder prevents flow of electric current, thus making the electricity-collecting property in the thickness direction of the electrode worse.